Overlay mark and method of measuring the same

ABSTRACT

A device having an overlay mark over a substrate and a method of adjusting multi-layer overlay alignment using the overlay mark for accuracy are disclosed. The overlay mark includes a first feature in a first layer, having a plurality of first alignment segments substantially parallel to each other extending only along an X direction; a second feature in a second layer over the first layer, having a plurality of second alignment segments substantially parallel to each other extending along a Y direction different from the X direction; and a third feature in a third layer over the second layer, having a plurality of third alignment segments substantially parallel to each other extending along the X direction and a plurality of fourth alignment segments substantially parallel to each other extending along the Y direction.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced rapidgrowth. Technological advances in IC materials and design have producedgenerations of ICs where each generation has smaller and more complexcircuits than the previous generation. Nowadays, the semiconductordevices and integrated circuits include multi-layer structures havingdimensions smaller than one micrometer. As known in the art, aphotolithography process is a step that determines the criticaldimension (CD) in the manufacture of a semiconductor integrated circuitdevice. Electric circuit patterns are formed by first transferring thepattern on a photo mask to a photoresist layer in a photolithographyprocess, and then transferring the pattern from the photoresist layer toan underlying material layer such as a dielectric layer or a metal layerin a subsequent etching process.

In addition to the control of CD, a successful photolithography processon a wafer includes alignment accuracy (AA). As the scaling downcontinues especially below 20 nm, aligning multiple layers accuratelyhas become more and more difficult. Therefore, the measurement ofaccuracy, i.e., the measurement of overlay error, is crucial to thesemiconductor fabrication process. An overlay mark is used as a tool formeasuring overlay error and to determine whether the photoresist patternis precisely aligned with the previous layer on a wafer after aphotolithography process.

If all or part of the mask is not aligned properly, the resultingfeatures may not align correctly with adjoining layers. This may resultin reduced device performance or complete device failure. While existingoverlay marks have been used to prevent this, they have not beenentirely satisfactory for small dimensional devices.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isemphasized that, in accordance with the standard practice in theindustry, various features are not drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion.

FIG. 1 is a top view of a wafer that illustrates positions of overlaymarks according to various aspects of the present disclosure.

FIG. 2 is a top view of the enlargement of a dotted region in FIG. 1.

FIG. 3 is a top view of an overlay mark for aligning different layersover a substrate according to various aspects of the present disclosure.

FIG. 4A is a cross-sectional view taken along a cutting line A-A′ ofFIG. 3.

FIG. 4B is a cross-sectional view taken along a cutting line B-B′ ofFIG. 3.

FIG. 5 is a top view of an overlay mark for aligning different layersover a substrate according to various aspects of the present disclosure.

FIG. 6A is a cross-sectional view taken along a cutting line C-C′ ofFIG. 5.

FIG. 6B is a cross-sectional view taken along a cutting line D-D′ ofFIG. 5.

FIG. 6C is a cross-sectional view taken along a cutting line E-E′ ofFIG. 5.

FIG. 6D is a cross-sectional view taken along a cutting line F-F′ ofFIG. 5.

FIG. 7 is a top view of an overlay mark for aligning different layersover a substrate according to various aspects of the present disclosure.

FIG. 8A is a cross-sectional view taken along a cutting line G-G′ ofFIG. 7.

FIG. 8B is a cross-sectional view taken along a cutting line H-H′ ofFIG. 7.

FIG. 9 is a top view of an overlay mark for aligning different layersover a substrate according to various aspects of the present disclosure.

FIG. 10A is a cross-sectional view taken along a cutting line K-K′ ofFIG. 9.

FIG. 10B is a cross-sectional view taken along a cutting line L-L′ ofFIG. 9.

FIG. 11 is a flowchart of a method of adjusting multi-layer overlayalignment according to various aspects of the present disclosure.

FIG. 12A is a cross-sectional view taken along a cutting line A-A′ ofFIG. 3, including two centerlines Y1 and Y2.

FIG. 12B is a cross-sectional view taken along a cutting line B-B′ ofFIG. 3, including two centerlines X1 and X2.

FIG. 13A is a signal waveform measured from the overlay mark in FIG.12A.

FIG. 13B is a signal waveform measured from the overlay mark in FIG.12B.

FIG. 14A shows the detail of sub-steps 412 a, 412 b, and 412 c of step412 in FIG. 11.

FIG. 14B shows the detail of sub-steps 408 a, 408 b, and 408 c of step408 in FIG. 11.

FIG. 15A is a cross-sectional view taken along a cutting line C-C′ ofFIG. 5, including a centerline Y2.

FIG. 15B is a cross-sectional view taken along a cutting line D-D′ ofFIG. 5, including a centerline X2.

FIG. 15C is a cross-sectional view taken along a cutting line E-E′ ofFIG. 5, including a centerline Y1.

FIG. 15D is a cross-sectional view taken along a cutting line F-F′ ofFIG. 5, including a centerline X1.

FIG. 16A is a signal waveform measured from the overlay mark in FIG.15A.

FIG. 16B is a signal waveform measured from the overlay mark in FIG.15B.

FIG. 16C is a signal waveform measured from the overlay mark in FIG.15C.

FIG. 16D is a signal waveform measured from the overlay mark in FIG.15D.

FIG. 17A is a cross-sectional view taken along a cutting line G-G′ ofFIG. 7, including an intensity I of an incident light and intensities I₃and I₄ of a first order diffraction.

FIG. 17B is a cross-sectional view taken along a cutting line H-H′ ofFIG. 7, including an intensity I of an incident light and intensities I₁and I₂ of a first order diffraction.

FIG. 18A shows the detail of sub-steps 412 d, 412 e, and 412 f of step412 in FIG. 11.

FIG. 18B shows the detail of sub-steps 408 d, 408 e, and 408 f of step408 in FIG. 11.

DETAILED DESCRIPTION

The present disclosure relates in general to an overlay mark forchecking alignment accuracy, and more particularly, to an overlay markfor aligning different layers on a substrate and to a method foradjusting multi-layer overlay alignment.

It is understood that the following disclosure provides many differentembodiments, or examples, for implementing different features of variousembodiments. Specific examples of components and arrangements aredescribed below to simplify the present disclosure. These are, ofcourse, merely examples and are not intended to be limiting. Forexample, the formation of a first feature over or on a second feature inthe description that follows may include embodiments in which the firstand second features are formed in direct contact, and may also includeembodiments in which additional features may be formed between the firstand second features, such that the first and second features may not bein direct contact. In addition, the present disclosure may repeatreference numerals and/or letters in the various examples. Thisrepetition is for the purpose of simplicity and clarity and does not initself dictate a relationship between the various embodiments and/orconfigurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. For example, if the device in the figures is turned over,elements described as being “below” or “beneath” other elements orfeatures would then be oriented “above” the other elements or features.Thus, the exemplary term “below” can encompass both an orientation ofabove and below. The apparatus may be otherwise oriented (rotated 90degrees or at other orientations) and the spatially relative descriptorsused herein may likewise be interpreted accordingly.

FIG. 1 is a top view of a wafer 20 that illustrates positions of overlaymarks according to various aspects of the present disclosure. FIG. 2 isa top view of the enlargement of a dotted region in FIG. 1. As shown inFIGS. 1 and 2, the wafer 20 is sawed along scribe lines 60 into aplurality of chips or dies 40. Normally, the overlay marks 80 arelocated on the scribe lines 60 at four corners of an edge of each chip40 or located inside each chip 40 to measure whether the present layer,such as an opening of a photoresist layer, is precisely aligned with apre-layer in the fabrication process.

While measuring an overlay error using an overlay mark according toprior approaches, an X-directional deviation is measured along astraight line in an X direction of the overlay mark. A Y-directionaldeviation is further measured along a straight line in a Y direction ofthe overlay mark. One single overlay mark can only be used to measureone X- and one Y-directional deviation between two layers on asubstrate. When all the overlay marks are measured using this method,whether the present layer and the pre-layer are precisely aligned can becalculated according to the X- and Y-directional deviations.

To check alignment accuracy between three layers, the prior approachused two separate overlay marks on the substrate. According to the priorapproach, one overlay mark is used for checking alignment accuracybetween the first pre-layer and the present layer, and the other overlaymark is used for checking alignment accuracy between the secondpre-layer and the present layer. The area cost is very high for twooverlay marks positioned on different areas of the substrate. As thedesign rule shrinks and the fabrication of the integrated circuits tendsto use multi-layer design, the area cost issue of the overlay marksbecomes seriously high. Additionally, for checking the alignmentaccuracy between the first pre-layer and the present layer, theY-directional deviation may be more concerned than the X-directionaldeviation. For checking the alignment accuracy between the secondpre-layer and the present layer, the X-directional deviation may be moreconcerned than the Y-directional deviation. However, the method formeasuring the overlay error according to the prior approach wouldcollect all the X-directional deviations and Y-directional deviationsbetween the first pre-layer and the present layer, and those between thesecond pre-layer and the present layer. It would be time-consuming tocollect and analyze some data which are not so concerned.

FIG. 3 is a top view of an overlay mark 100 for aligning differentlayers over a substrate 160 according to various aspects of the presentdisclosure. As shown in FIG. 3, a device (not shown) comprises theoverlay mark 100 over the substrate 160. The device may include variousdevices or elements, such as semiconductor devices, bipolar junctiontransistors, resistors, capacitors, diodes, fuses, etc., but issimplified for a better understanding of the concepts of the presentdisclosure. The substrate 160 may typically be a silicon substrate. Thesubstrate 160 may include various doping configurations depending ondesign requirements as known in the art. The substrate 160 may alsoinclude other elementary semiconductors such as germanium and diamond.Alternatively, the substrate 160 may include a compound semiconductorand/or an alloy semiconductor. Further, the substrate 160 may optionallyinclude an epitaxial layer (epi layer), may be strained for performanceenhancement, may include a silicon-on-insulator (SOI) structure, and/orhave other suitable enhancement elements. The overlay mark 100 comprisesvarious features over the substrate 160, such as a first feature 130, asecond feature 140, and a third feature 120. The first feature 130represents the pattern of a first pre-layer (hereinafter, a first layer170), the second feature 140 represents the pattern of a secondpre-layer (hereinafter, a second layer 180), and the third feature 120represents the pattern of the present layer (hereinafter, a third layer190), such as an opening of a mask 110.

FIG. 4A is a cross-sectional view taken along a cutting line A-A′ ofFIG. 3. As shown in FIGS. 3 and 4A, the first feature 130 is disposed ina first layer 170 over the substrate 160, and the first feature 130extends only along a first longitudinal direction. The first feature 130comprises a plurality of alignment segments 130 a, 130 b substantiallyparallel to each other. The first feature 130 may comprise an isolationstructure such as a shallow trench isolation (STI), a field oxide (FOX),a local-oxidation of silicon (LOCOS) feature, and/or other suitableisolation element. The isolation structure may comprise a dielectricmaterial such as silicon oxide, silicon nitride, silicon oxy-nitride,fluoride-doped silicate (FSG), a low-k dielectric material, combinationsthereof, and/or other suitable material. In some embodiments, the firstlongitudinal direction is an X direction.

FIG. 4B is a cross-sectional view taken along a cutting line B-B′ ofFIG. 3. As shown in FIGS. 3 and 4B, the second feature 140 is disposedin a second layer 180 over the first layer 170, and the second feature140 extends along a second longitudinal direction different from thefirst longitudinal direction. The second feature 140 comprises aplurality of alignment segments 140 a, 140 b substantially parallel toeach other. The second feature 140 may comprise a gate electrode. Thegate electrode may be sacrificial, for example, such as formed in areplacement gate process. In some embodiments, the gate electrodeincludes polysilicon. The polysilicon may be formed by suitabledeposition processes such as, for example, low-pressure chemical vapordeposition (LPCVD) and plasma-enhanced CVD (PECVD). In some embodiments,the gate electrode includes conductive material such as metal. In someembodiments, the second longitudinal direction is a Y directionsubstantially perpendicular to the X direction.

As depicted in FIGS. 3, 4A, and 4B, the third feature 120 is disposed ina third layer 190 over the second layer 180, and the third feature 120extends along both the first longitudinal direction and the secondlongitudinal direction. The third feature 120 comprises a plurality ofalignment segments 120 a, 120 b substantially parallel to each otherextending along the first longitudinal direction and a plurality ofalignment segments 120 c, 120 d substantially parallel to each otherextending along the second longitudinal direction. The third feature 120may comprise a continuous opening or a plurality of openings 120 a-120 din a mask 110. The mask 110 may comprise a positive-tone ornegative-tone photoresist such as polymer, or a hard mask such assilicon nitride or silicon oxy-nitride. The third layer 190 may bepatterned using suitable photolithography processes such as, forexample, forming a photoresist layer, exposing the photoresist layer toa pattern, baking and developing the photoresist to form the mask 110.The mask 110 may then be used to etch a pattern (e.g., a contact) into adielectric layer 150 such as silicon oxide disposed below the mask 110.In some embodiments, the third feature 120 comprises four alignmentsegments 120 a-120 d forming a rectangular pattern or a square pattern.In some embodiments, the first longitudinal direction is substantiallyperpendicular to the second longitudinal direction. In some embodiments,the first longitudinal direction is the X direction and the secondlongitudinal direction is the Y direction substantially perpendicular tothe X direction.

FIG. 5 is a top view of an overlay mark 200 for aligning differentlayers over a substrate 260 according to various aspects of the presentdisclosure. As shown in FIG. 5, a device (not shown) comprises theoverlay mark 200 over the substrate 260. The device may include variousdevices or elements, such as semiconductor devices, bipolar junctiontransistors, resistors, capacitors, diodes, fuses, etc., but issimplified for a better understanding of the concepts of the presentdisclosure. The substrate 260 may typically be a silicon substrate. Thesubstrate 260 may include various doping configurations depending ondesign requirements as known in the art. The substrate 260 may alsoinclude other elementary semiconductors such as germanium and diamond.Alternatively, the substrate 260 may include a compound semiconductorand/or an alloy semiconductor. Further, the substrate 260 may optionallyinclude an epitaxial layer (epi layer), may be strained for performanceenhancement, may include a silicon-on-insulator (SOI) structure, and/orhave other suitable enhancement elements. The overlay mark 200 comprisesvarious features over the substrate 260, such as a first feature 230, asecond feature 240, and a third feature 220. The first feature 230represents the pattern of a first pre-layer (hereinafter, a first layer270), the second feature 240 represents the pattern of a secondpre-layer (hereinafter, a second layer 280), and the third feature 220represents the pattern of the present layer (hereinafter, a third layer290), such as an opening of a mask 210.

FIG. 6A is a cross-sectional view taken along a cutting line C-C′ ofFIG. 5. Please note that the cross-sectional view in FIG. 6A shows onlythe dashed portions of the cutting line C-C′ in FIG. 5 while the dottedportion is not shown. In other words, the cross-sectional views of thedashed portions of the cutting line C-C′ are linked together in FIG. 6A.As shown in FIGS. 5 and 6A, the first feature 230 is disposed in a firstlayer 270 over the substrate 260, and the first feature 230 extends onlyalong a first longitudinal direction. The first feature 230 comprises aplurality of alignment segments 230 a-230 t substantially parallel toeach other. The first feature 230 may comprise an isolation structuresuch as a shallow trench isolation (STI), a field oxide (FOX), alocal-oxidation of silicon (LOCOS) feature, and/or other suitableisolation element. The isolation structure may comprise a dielectricmaterial such as silicon oxide, silicon nitride, silicon oxy-nitride,fluoride-doped silicate (FSG), a low-k dielectric material, combinationsthereof, and/or other suitable material. In some embodiments, the firstlongitudinal direction is an X direction.

FIG. 6B is a cross-sectional view taken along a cutting line D-D′ ofFIG. 5. Similarly, the cross-sectional view in FIG. 6B shows only thedashed portions of the cutting line D-D′ in FIG. 5 while the dottedportion is not shown. As shown in FIGS. 5 and 6B, the second feature 240is disposed in a second layer 280 over the first layer 270, and thesecond feature 240 extends along a second longitudinal directiondifferent from the first longitudinal direction. The second feature 240comprises a plurality of alignment segments 240 a-240 t substantiallyparallel to each other. The second feature 240 may comprise a gateelectrode. The gate electrode may be sacrificial, for example, such asformed in a replacement gate process. In some embodiments, the gateelectrode includes polysilicon. The polysilicon may be formed bysuitable deposition processes such as, for example, low-pressurechemical vapor deposition (LPCVD) and plasma-enhanced CVD (PECVD). Insome embodiments, the gate electrode includes conductive material suchas metal. In some embodiments, the second longitudinal direction is a Ydirection substantially perpendicular to the X direction.

FIG. 6C is a cross-sectional view taken along a cutting line E-E′ ofFIG. 5. The cross-sectional view in FIG. 6C shows only the dashedportions of the cutting line E-E′ in FIG. 5 while the dotted portion isnot shown. FIG. 6D is a cross-sectional view taken along a cutting lineF-F′ of FIG. 5. The cross-sectional view in FIG. 6D shows only thedashed portions of the cutting line F-F′ in FIG. 5 while the dottedportion is not shown. As shown in FIGS. 5, 6C, and 6D, the third feature220 is disposed in a third layer 290 over the second layer 280, and thethird feature 220 extends along both the first longitudinal directionand the second longitudinal direction. The third feature 220 comprises aplurality of alignment segments 220 a-220 t substantially parallel toeach other extending along the first longitudinal direction and aplurality of alignment segments 220 a′-220 t′ substantially parallel toeach other extending along the second longitudinal direction. The thirdfeature 220 may comprise a plurality of openings 220 a-220 t and 220a′-220 t′ in a mask 210. The mask 210 may comprise a positive-tone ornegative-tone photoresist such as polymer, or a hard mask such assilicon nitride or silicon oxy-nitride. The third layer 290 may bepatterned using suitable photolithography processes such as, forexample, forming a photoresist layer, exposing the photoresist layer toa pattern, baking and developing the photoresist to form the mask 210.The mask 210 may then be used to etch a pattern (e.g., a contact) into adielectric layer 250 such as silicon oxide disposed below the mask 210.In some embodiments, the first longitudinal direction is substantiallyperpendicular to the second longitudinal direction. In some embodiments,the first longitudinal direction is the X direction and the secondlongitudinal direction is the Y direction substantially perpendicular tothe X direction.

FIG. 7 is a top view of an overlay mark 300 for aligning differentlayers over a substrate 360 according to various aspects of the presentdisclosure. As shown in FIG. 7, a device (not shown) comprises theoverlay mark 300 over the substrate 360. The device may include variousdevices or elements, such as semiconductor devices, bipolar junctiontransistors, resistors, capacitors, diodes, fuses, etc., but issimplified for a better understanding of the concepts of the presentdisclosure. The substrate 360 may typically be a silicon substrate. Thesubstrate 360 may include various doping configurations depending ondesign requirements as known in the art. The substrate 360 may alsoinclude other elementary semiconductors such as germanium and diamond.Alternatively, the substrate 360 may include a compound semiconductorand/or an alloy semiconductor. Further, the substrate 360 may optionallyinclude an epitaxial layer (epi layer), may be strained for performanceenhancement, may include a silicon-on-insulator (SOI) structure, and/orhave other suitable enhancement elements. The overlay mark 300 comprisesvarious features over the substrate 360, such as a first feature 330, asecond feature 340, and a third feature 320. The first feature 330represents the pattern of a first pre-layer (hereinafter, a first layer370), the second feature 340 represents the pattern of a secondpre-layer (hereinafter, a second layer 380), and the third feature 320represents the pattern of the present layer (hereinafter, a third layer390), such as an opening of a mask 310.

FIG. 8A is a cross-sectional view taken along a cutting line G-G′ ofFIG. 7. The cross-sectional view in FIG. 8A shows only the dashedportions of the cutting line G-G′ in FIG. 7 while the dotted portion isnot shown. As shown in FIGS. 7 and 8A, the first feature 330 is disposedin a first layer 370 over the substrate 360, and the first feature 330extends only along a first longitudinal direction. The first feature 330comprises a plurality of alignment segments 330 a-330 j substantiallyparallel to each other. The first feature 330 may comprise an isolationstructure such as a shallow trench isolation (STI), a field oxide (FOX),a local-oxidation of silicon (LOCOS) feature, and/or other suitableisolation element. The isolation structure may comprise a dielectricmaterial such as silicon oxide, silicon nitride, silicon oxy-nitride,fluoride-doped silicate (FSG), a low-k dielectric material, combinationsthereof, and/or other suitable material. In some embodiments, the firstlongitudinal direction is an X direction.

FIG. 8B is a cross-sectional view taken along a cutting line H-H′ ofFIG. 7. The cross-sectional view in FIG. 8B shows only the dashedportions of the cutting line H-H′ in FIG. 7 while the dotted portion isnot shown. As shown in FIGS. 7 and 8B, the second feature 340 isdisposed in a second layer 380 over the first layer 370, and the secondfeature 340 extends along a second longitudinal direction different fromthe first longitudinal direction. The second feature 340 comprises aplurality of alignment segments 340 a-340 j substantially parallel toeach other. The second feature 340 may comprise a gate electrode. Thegate electrode may be sacrificial, for example, such as formed in areplacement gate process. In some embodiments, the gate electrodeincludes polysilicon. The polysilicon may be formed by suitabledeposition processes such as, for example, low-pressure chemical vapordeposition (LPCVD) and plasma-enhanced CVD (PECVD). In some embodiments,the gate electrode includes conductive material such as metal. In someembodiments, the second longitudinal direction is a Y directionsubstantially perpendicular to the X direction.

As depicted in FIGS. 7, 8A, and 8B, the third feature 320 is disposed ina third layer 390 over the second layer 380, and the third feature 320extends along both the first longitudinal direction and the secondlongitudinal direction. The third feature 320 comprises a plurality ofalignment segments 320 a-320 j substantially parallel to each otherextending along the first longitudinal direction and a plurality ofalignment segments 320 a′-320 j′ substantially parallel to each otherextending along the second longitudinal direction. The third feature 320may comprise a plurality of openings 320 a-320 j and 320 a′-320 j′ in amask 310. The mask 310 may comprise a positive-tone or negative-tonephotoresist such as polymer, or a hard mask such as silicon nitride orsilicon oxy-nitride. The third layer 390 may be patterned using suitablephotolithography processes such as, for example, forming a photoresistlayer, exposing the photoresist layer to a pattern, baking anddeveloping the photoresist to form the mask 310. The mask 310 may thenbe used to etch a pattern (e.g., a contact) into a dielectric layer 350such as silicon oxide disposed below the mask 310. In some embodiments,the first longitudinal direction is substantially perpendicular to thesecond longitudinal direction. In some embodiments, the firstlongitudinal direction is the X direction and the second longitudinaldirection is the Y direction substantially perpendicular to the Xdirection.

The overlay marks of the present disclosure are not limited to theabove-mentioned embodiments, and may have other different embodiments.To simplify the description and for the convenience of comparisonbetween each of the embodiments of the present disclosure, the identicalcomponents in each of the following embodiments are marked withidentical numerals. For making it easier to compare the differencebetween the embodiments, the following description will detail thedissimilarities among different embodiments and the identical featureswill not be redundantly described.

FIG. 9 is a top view of an overlay mark 305 for aligning differentlayers over a substrate 360 according to various aspects of the presentdisclosure. FIG. 9 is similar to FIG. 7 except that the shape of theoverlay mark 305 is rectangular compared to that of the overlay mark 300being square. The overlay mark 305 may be located on the scribe lines atthe four corners of the edge of each chip to save the area, or locatedinside each chip. FIG. 10A is a cross-sectional view taken along acutting line K-K′ of FIG. 9. FIG. 10A is similar to FIG. 8A, and thecross-sectional view in FIG. 10A shows only the dashed portions of thecutting line K-K′ in FIG. 9 while the dotted portion is not shown. FIG.10B is a cross-sectional view taken along a cutting line L-L′ of FIG. 9.FIG. 10B is similar to FIG. 8B, and the cross-sectional view in FIG. 10Bshows only the dashed portions of the cutting line L-L′ in FIG. 9 whilethe dotted portion is not shown. The detailed description of FIGS. 9,10A, and 10B for the overlay mark 305 may refer to that of FIGS. 7, 8A,and 8B for the overlay mark 300.

FIG. 11 is a flowchart of a method 400 of adjusting multi-layer overlayalignment according to various aspects of the present disclosure. It isunderstood that additional steps can be provided before, during, andafter the method 400, and some of the steps described can be replaced oreliminated for other embodiments of the method. The method 400 begins atstep 402 in which a first feature in a first layer over a substrate isprovided, wherein the first feature comprises a plurality of firstalignment segments substantially parallel to each other extending onlyalong an X direction. The method 400 continues with step 404 in which asecond feature in a second layer over the first layer is provided,wherein the second feature comprises a plurality of second alignmentsegments substantially parallel to each other extending along a Ydirection different from the X direction. The method 400 continues withstep 406 in which a third feature in a third layer over the second layeris provided, wherein the third feature comprises a plurality of thirdalignment segments substantially parallel to each other extending alongthe X direction and a plurality of fourth alignment segmentssubstantially parallel to each other extending along the Y direction.The method 400 continues with step 408 in which an X-directionaldeviation between the fourth alignment segments and the second alignmentsegments is measured. The method 400 continues with step 410 in which anX-directional offset value by the X-directional deviation is calculated.The method 400 continues with step 412 in which a Y-directionaldeviation between the third alignment segments and the first alignmentsegments is measured. The method 400 continues with step 414 in which aY-directional offset value by the Y-directional deviation is calculated.The method 400 continues with step 416 in which the X-directional offsetvalue or the Y-directional offset value is used to compensate for anoverlay error. The method 400 may further comprise repeating aphotolithography process if the overlay error is larger than anacceptable deviation. The discussion that follows illustratesembodiments of overlay marks that can be measured according to themethod 400 of FIG. 11.

As depicted in FIGS. 3 and 4A, and step 402 in FIG. 11, the method 400begins at step 402 by providing a first feature 130 in a first layer 170over a substrate 160, wherein the first feature 130 comprises aplurality of first alignment segments 130 a, 130 b substantiallyparallel to each other extending only along an X direction. The firstfeature 130 may comprise an isolation structure such as a shallow trenchisolation (STI), a field oxide (FOX), a local-oxidation of silicon(LOCOS) feature, and/or other suitable isolation element. The isolationstructure may comprise a dielectric material such as silicon oxide,silicon nitride, silicon oxy-nitride, fluoride-doped silicate (FSG), alow-k dielectric material, combinations thereof, and/or other suitablematerial.

As depicted in FIGS. 3 and 4B, and step 404 in FIG. 11, the method 400continues with step 404 by providing a second feature 140 in a secondlayer 180 over the first layer 170, wherein the second feature 140comprises a plurality of second alignment segments 140 a, 140 bsubstantially parallel to each other extending along a Y directiondifferent from the X direction. The second feature 140 may comprise agate electrode. The gate electrode may be sacrificial, for example, suchas formed in a replacement gate process. In some embodiments, the gateelectrode includes polysilicon. The polysilicon may be formed bysuitable deposition processes such as, for example, low-pressurechemical vapor deposition (LPCVD) and plasma-enhanced CVD (PECVD). Insome embodiments, the gate electrode includes conductive material suchas metal. In some embodiments, the Y direction is substantiallyperpendicular to the X direction.

As depicted in FIGS. 3, 4A, and 4B, and step 406 in FIG. 11, the method400 continues with step 406 by providing a third feature 120 in a thirdlayer 190 over the second layer 180, wherein the third feature 120comprises a plurality of third alignment segments 120 a, 120 bsubstantially parallel to each other extending along the X direction anda plurality of fourth alignment segments 120 c, 120 d substantiallyparallel to each other extending along the Y direction. The thirdfeature 120 may comprise a continuous opening or a plurality of openings120 a-120 d in a mask 110. The mask 110 may comprise a positive-tone ornegative-tone photoresist such as polymer, or a hard mask such assilicon nitride or silicon oxy-nitride. The third layer 190 may bepatterned using suitable photolithography processes such as, forexample, forming a photoresist layer, exposing the photoresist layer toa pattern, baking and developing the photoresist to form the mask 110.The mask 110 may then be used to etch a pattern (e.g., a contact) into adielectric layer 150 such as silicon oxide disposed below the mask 110.In some embodiments, the third feature 120 comprises four alignmentsegments 120 a-120 d forming a rectangular pattern or a square pattern.In some embodiments, the X direction is substantially perpendicular tothe Y direction.

FIG. 12B is a cross-sectional view taken along a cutting line B-B′ ofFIG. 3, including two centerlines X1 and X2. FIG. 13B is a signalwaveform measured from the overlay mark 100 in FIG. 12B. As depicted inFIGS. 12B and 13B, and step 408 in FIG. 11, the method 400 continueswith step 408 by measuring an X-directional deviation (ΔX) between thefourth alignment segments 120 c, 120 d and the second alignment segments140 a, 140 b. Peak signals of the second alignment segments 140 a, 140 bin FIG. 12B are denoted as 142 a and 142 b in FIG. 13B, and the peaksignals of the fourth alignment segments 120 c, 120 d in FIG. 12B aredenoted as 122 c and 122 d in FIG. 13B. Using the overlay mark 100 tomeasure the alignment accuracy, a first mean value 122′ of the peaksignals 122 c and 122 d is obtained. A second mean value 142 of the peaksignals 142 a and 142 b is also obtained. A difference between the firstmean value 122′ and the second mean value 142 is calculated as theX-directional deviation (ΔX) between the fourth alignment segments 120c, 120 d and the second alignment segments 140 a, 140 b.

In some embodiments, step 408 may comprise several sub-steps 408 a, 408b, and 408 c. FIG. 14B shows the detail of sub-steps 408 a-408 c of step408 in FIG. 11. As depicted in FIGS. 12B and 13B, and sub-steps 408a-408 c in FIG. 14B, the first mean value 122′ defines the centerline X1parallel to the fourth alignment segments 120 c, 120 d. The second meanvalue 142 defines the centerline X2 parallel to the second alignmentsegments 140 a, 140 b. The difference between the first mean value 122′and the second mean value 142 is calculated as the X-directionaldeviation (ΔX) between the centerline X1 and the centerline X2. Asdepicted in step 410 in FIG. 11, the method 400 continues with step 410by calculating an X-directional offset value by the X-directionaldeviation (ΔX). The X-directional offset value is the reverse of theX-directional deviation (ΔX), or any value appropriate for adjusting theX-directional deviation (ΔX).

FIG. 12A is a cross-sectional view taken along a cutting line A-A′ ofFIG. 3, including two centerlines Y1 and Y2. FIG. 13A is a signalwaveform measured from the overlay mark 100 in FIG. 12A. As depicted inFIGS. 12A and 13A, and step 412 in FIG. 11, the method 400 continueswith step 412 by measuring a Y-directional deviation (ΔY) between thethird alignment segments 120 a, 120 b and the first alignment segments130 a, 130 b. The peak signals of the first alignment segments 130 a,130 b in FIG. 12A are denoted as 132 a and 132 b in FIG. 13A, and thepeak signals of the third alignment segments 120 a, 120 b in FIG. 12Aare denoted as 122 a and 122 b in FIG. 13A. Using the overlay mark 100to measure the alignment accuracy, a third mean value 122 of the peaksignals 122 a and 122 b is obtained. A fourth mean value 132 of the peaksignals 132 a and 132 b is also obtained. A difference between the thirdmean value 122 and the fourth mean value 132 is calculated as theY-directional deviation (ΔY) between the third alignment segments 120 a,120 b and the first alignment segments 130 a, 130 b.

In some embodiments, step 412 may comprise several sub-steps 412 a, 412b, and 412 c. FIG. 14A shows the detail of sub-steps 412 a-412 c of step412 in FIG. 11. As depicted in FIGS. 12A and 13A, and sub-steps 412a-412 c in FIG. 14A, the third mean value 122 defines the centerline Y1parallel to the third alignment segments 120 a, 120 b. The fourth meanvalue 132 defines the centerline Y2 parallel to the first alignmentsegments 130 a, 130 b. The difference between the third mean value 122and the fourth mean value 132 is calculated as the Y-directionaldeviation (ΔY) between the centerline Y1 and the centerline Y2. Asdepicted in step 414 in FIG. 11, the method 400 continues with step 414by calculating a Y-directional offset value by the Y-directionaldeviation (ΔY). The Y-directional offset value is the reverse of theY-directional deviation (ΔY), or any value appropriate for adjusting theY-directional deviation (ΔY).

As depicted in step 416 in FIG. 11, the method 400 continues with step416 by using the X-directional offset value and/or the Y-directionaloffset value to compensate for an overlay error. The overlay error maycomprise the X-directional deviation (ΔX), the Y-directional deviation(ΔY), or the combination of both. After the overlay error iscompensated, the multi-layer overlay alignment accuracy (AA) is adjustedand will be implemented in next run of the photolithography process. Insome embodiments, the method 400 may further comprise repeating aphotolithography process if the overlay error is larger than anacceptable deviation. If the overlay error is larger than an acceptabledeviation, the alignment between the third feature 120 and the firstfeature 130, and/or the alignment between the third feature 120 and thesecond feature 140, do/does not reach the required accuracy.Consequently, the third feature 120 has to be removed, and thephotolithography process has to be repeated until the overlay error isno larger than the acceptable deviation.

In various embodiments, the method 400 may be used with the overlay mark200 of FIGS. 5 and 6A-6D. As depicted in FIGS. 5 and 6A, and step 402 inFIG. 11, the method 400 begins at step 402 by providing a first feature230 in a first layer 270 over a substrate 260, wherein the first feature230 comprises a plurality of first alignment segments 230 a-230 tsubstantially parallel to each other extending only along an Xdirection. The first feature 230 may comprise an isolation structuresuch as a shallow trench isolation (STI), a field oxide (FOX), alocal-oxidation of silicon (LOCOS) feature, and/or other suitableisolation element. The isolation structure may comprise a dielectricmaterial such as silicon oxide, silicon nitride, silicon oxy-nitride,fluoride-doped silicate (FSG), a low-k dielectric material, combinationsthereof, and/or other suitable material.

As depicted in FIGS. 5 and 6B, and step 404 in FIG. 11, the method 400continues with step 404 by providing a second feature 240 in a secondlayer 280 over the first layer 270, wherein the second feature 240comprises a plurality of second alignment segments 240 a-240 tsubstantially parallel to each other extending along a Y directiondifferent from the X direction. The second feature 240 may comprise agate electrode. The gate electrode may be sacrificial, for example, suchas formed in a replacement gate process. In some embodiments, the gateelectrode includes polysilicon. The polysilicon may be formed bysuitable deposition processes such as, for example, low-pressurechemical vapor deposition (LPCVD) and plasma-enhanced CVD (PECVD). Insome embodiments, the gate electrode includes conductive material suchas metal. In some embodiments, the Y direction is substantiallyperpendicular to the X direction.

As depicted in FIGS. 5, 6C, and 6D, and step 406 in FIG. 11, the method400 continues with step 406 by providing a third feature 220 in a thirdlayer 290 over the second layer 280, wherein the third feature 220comprises a plurality of third alignment segments 220 a-220 tsubstantially parallel to each other extending along the X direction anda plurality of fourth alignment segments 220 a′-220 t′ substantiallyparallel to each other extending along the Y direction. The thirdfeature 220 may comprise a plurality of openings 220 a-220 t and 220a′-220 t′ in a mask 210. The mask 210 may comprise a positive-tone ornegative-tone photoresist such as polymer, or a hard mask such assilicon nitride or silicon oxy-nitride. The third layer 290 may bepatterned using suitable photolithography processes such as, forexample, forming a photoresist layer, exposing the photoresist layer toa pattern, baking and developing the photoresist to form the mask 210.The mask 210 may then be used to etch a pattern (e.g., a contact) into adielectric layer 250 such as silicon oxide disposed below the mask 210.In some embodiments, the X direction is substantially perpendicular tothe Y direction.

FIG. 15B is a cross-sectional view taken along a cutting line D-D′ ofFIG. 5, including a centerline X2, and FIG. 16B is a signal waveformmeasured from the overlay mark 200 in FIG. 15B. FIG. 15D is across-sectional view taken along a cutting line F-F′ of FIG. 5,including a centerline X1, and FIG. 16D is a signal waveform measuredfrom the overlay mark 200 in FIG. 15D. As depicted in FIGS. 15B, 15D,16B, and 16D, and step 408 in FIG. 11, the method 400 continues withstep 408 by measuring an X-directional deviation (ΔX) between the fourthalignment segments 220 a′-220 t′ and the second alignment segments 240a-240 t. The peak signals of the second alignment segments 240 a-240 tin FIG. 15B are denoted as 242 a-242 t in FIG. 16B, and the peak signalsof the fourth alignment segments 220 a′-220 t′ in FIG. 15D are denotedas 222 a′-222 t′ in FIG. 16D. Using the overlay mark 200 to measure thealignment accuracy, a first mean value 222′ of the peak signals 222a′-222 t′ is obtained. A second mean value 242 of the peak signals 242a-242 t is also obtained. A difference between the first mean value 222′and the second mean value 242 is calculated as the X-directionaldeviation (ΔX) between the fourth alignment segments 220 a′-220 t′ andthe second alignment segments 240 a-240 t.

In some embodiments, step 408 may comprise several sub-steps 408 a, 408b, and 408 c. FIG. 14B shows the detail of sub-steps 408 a-408 c of step408 in FIG. 11. As depicted in FIGS. 15B, 15D, 16B, and 16D, andsub-steps 408 a-408 c in FIG. 14B, the first mean value 222′ defines thecenterline X1 parallel to the fourth alignment segments 220 a′-220 t′.The second mean value 242 defines the centerline X2 parallel to thesecond alignment segments 240 a-240 t. The difference between the firstmean value 222′ and the second mean value 242 is calculated as theX-directional deviation (ΔX) between the centerline X1 and thecenterline X2. As depicted in step 410 in FIG. 11, the method 400continues with step 410 by calculating an X-directional offset value bythe X-directional deviation (ΔX). The X-directional offset value is thereverse of the X-directional deviation (ΔX), or any value appropriatefor adjusting the X-directional deviation (ΔX).

FIG. 15A is a cross-sectional view taken along a cutting line C-C′ ofFIG. 5, including a centerline Y2, and FIG. 16A is a signal waveformmeasured from the overlay mark 200 in FIG. 15A. FIG. 15C is across-sectional view taken along a cutting line E-E′ of FIG. 5,including a centerline Y1, and FIG. 16C is a signal waveform measuredfrom the overlay mark 200 in FIG. 15C. As depicted in FIGS. 15A, 15C,16A, and 16C, the method 400 continues with step 412 by measuring aY-directional deviation (ΔY) between the third alignment segments 220a-220 t and the first alignment segments 230 a-230 t. The peak signalsof the first alignment segments 230 a-230 t in FIG. 15A are denoted as232 a-232 t in FIG. 16A, and the peak signals of the third alignmentsegments 220 a-220 t in FIG. 15C are denoted as 222 a-222 t in FIG. 16C.Using the overlay mark 200 to measure the alignment accuracy, a thirdmean value 222 of the peak signals 222 a-222 t is obtained. A fourthmean value 232 of the peak signals 232 a-232 t is also obtained. Adifference between the third mean value 222 and the fourth mean value232 is calculated as the Y-directional deviation (ΔY) between the thirdalignment segments 220 a-220 t and the first alignment segments 230a-230 t.

In some embodiments, step 412 may comprise several sub-steps 412 a, 412b, and 412 c. FIG. 14A shows the detail of sub-steps 412 a-412 c of step412 in FIG. 11. As depicted in FIGS. 15A, 15C, 16A, and 16C, andsub-steps 412 a-412 c in FIG. 14A, the third mean value 222 defines thecenterline Y1 parallel to the third alignment segments 220 a-220 t. Thefourth mean value 232 defines the centerline Y2 parallel to the firstalignment segments 230 a-230 t. The difference between the third meanvalue 222 and the fourth mean value 232 is calculated as theY-directional deviation (ΔY) between the centerline Y1 and thecenterline Y2. As depicted in step 414 in FIG. 11, the method 400continues with step 414 by calculating a Y-directional offset value bythe Y-directional deviation (ΔY). The Y-directional offset value is thereverse of the Y-directional deviation (ΔY), or any value appropriatefor adjusting the Y-directional deviation (ΔY).

As depicted in step 416 in FIG. 11, the method 400 continues with step416 by using the X-directional offset value and/or the Y-directionaloffset value to compensate for an overlay error. The overlay error maycomprise the X-directional deviation (ΔX), the Y-directional deviation(ΔY), or the combination of both. After the overlay error iscompensated, the multi-layer overlay alignment accuracy (AA) is adjustedand will be implemented in next run of the photolithography process. Insome embodiments, the method 400 may further comprise repeating aphotolithography process if the overlay error is larger than anacceptable deviation. If the overlay error is larger than an acceptabledeviation, the alignment between the third feature 220 and the firstfeature 230, and/or the alignment between the third feature 220 and thesecond feature 240, do/does not reach the required accuracy.Consequently, the third feature 220 has to be removed, and thephotolithography process has to be repeated until the overlay error isno larger than the acceptable deviation.

In various embodiments, the method 400 may be used with the overlay mark300 of FIGS. 7, 8A, and 8B. As depicted in FIGS. 7 and 8A, and step 402in FIG. 11, the method 400 begins at step 402 by providing a firstfeature 330 in a first layer 370 over a substrate 360, wherein the firstfeature 330 comprises a plurality of first alignment segments 330 a-330j substantially parallel to each other extending only along an Xdirection. The first feature 330 may comprise an isolation structuresuch as a shallow trench isolation (STI), a field oxide (FOX), alocal-oxidation of silicon (LOCOS) feature, and/or other suitableisolation element. The isolation structure may comprise a dielectricmaterial such as silicon oxide, silicon nitride, silicon oxy-nitride,fluoride-doped silicate (FSG), a low-k dielectric material, combinationsthereof, and/or other suitable material.

As depicted in FIGS. 7 and 8B, and step 404 in FIG. 11, the method 400continues with step 404 by providing a second feature 340 in a secondlayer 380 over the first layer 370, wherein the second feature 340comprises a plurality of second alignment segments 340 a-340 jsubstantially parallel to each other extending along a Y directiondifferent from the X direction. The second feature 340 may comprise agate electrode. The gate electrode may be sacrificial, for example, suchas formed in a replacement gate process. In some embodiments, the gateelectrode includes polysilicon. The polysilicon may be formed bysuitable deposition processes such as, for example, low-pressurechemical vapor deposition (LPCVD) and plasma-enhanced CVD (PECVD). Insome embodiments, the gate electrode includes conductive material suchas metal. In some embodiments, the Y direction is substantiallyperpendicular to the X direction.

As depicted in FIGS. 7, 8A, and 8B, and step 406 in FIG. 11, the method400 continues with step 406 by providing a third feature 320 in a thirdlayer 390 over the second layer 380, wherein the third feature 320comprises a plurality of third alignment segments 320 a-320 jsubstantially parallel to each other extending along the X direction anda plurality of fourth alignment segments 320 a′-320 j′ substantiallyparallel to each other extending along the Y direction. The thirdfeature 320 may comprise a plurality of openings 320 a-320 j and 320a′-320 j′ in a mask 310. The mask 310 may comprise a positive-tone ornegative-tone photoresist such as polymer, or a hard mask such assilicon nitride or silicon oxy-nitride. The third layer 390 may bepatterned using suitable photolithography processes such as, forexample, forming a photoresist layer, exposing the photoresist layer toa pattern, baking and developing the photoresist to form the mask 310.The mask 310 may then be used to etch a pattern (e.g., a contact) into adielectric layer 350 such as silicon oxide disposed below the mask 310.In some embodiments, the X direction is substantially perpendicular tothe Y direction.

FIG. 17B is a cross-sectional view taken along a cutting line H-H′ ofFIG. 7, including an intensity I of an incident light and intensities I₁and I₂ of a first order diffraction. As depicted in FIG. 17B and step408 in FIG. 11, the method 400 continues with step 408 by measuring anX-directional deviation (ΔX) between the fourth alignment segments 320a′-320 j′ and the second alignment segments 340 a-340 j. The overlaymeasurement principle used here is based on measuring asymmetry via thedetection of first or higher order diffraction. In some embodiments,step 408 may comprise several sub-steps 408 d, 408 e, and 408 f. FIG.18B shows the detail of sub-steps 408 d-408 f of step 408 in FIG. 11. Asdepicted in FIG. 17B, and sub-steps 408 d-408 f in FIG. 18B, anintensity I₁ of a positive first order diffraction on gratings of thefourth alignment segments 320 a′-320 j′ and the second alignmentsegments 340 a-340 j is detected. An intensity I₂ of a negative firstorder diffraction on gratings of the fourth alignment segments 320a′-320 j′ and the second alignment segments 340 a-340 j, is alsodetected. An intensity difference of I₁ and I₂ on gratings of the fourthalignment segments 320 a′-320 j′ and the second alignment segments 340a-340 j is calculated, wherein the X-directional deviation (ΔX) isproportional to the intensity difference.

If gratings of the fourth alignment segments 320 a′-320 j′ and thesecond alignment segments 340 a-340 j were on top of each other, formingone grating, the intensity I₁ of the positive first order diffractionand the intensity I₂ of the negative first order diffraction would bethe same, then there would be no X-directional deviation (ΔX) betweenthe fourth alignment segments 320 a′-320 j′ and the second alignmentsegments 340 a-340 j. If there is any misalignment between these twogratings, this will induce an asymmetry A (i.e. the intensitydifference) between the intensities of I₁ and I₂, which is proportionalto the X-directional deviation (ΔX). This relationship could beexpressed as one equation A=I₁-I₂=K*ΔX. In order to determine ΔX, theproportionality factor K needs to be determined first. To solve thisproblem, gratings are shifted with respect to each other by a distanced. If there was no ΔX, there would be gratings with a relative shift +dyielding an asymmetry A₁=K*d and gratings with a relative shift −dyielding an asymmetry A₂=−K*d. In the presence of ΔX, the relativeshifts of gratings amount to ΔX+d and ΔX−d, yielding asymmetriesA₁=K*(ΔX+d) and A₂=K*(ΔX−d). With these two equations, one may findΔX=d*(A₁+A₂)/(A₁-A₂). So the X-directional deviation (ΔX) is obtained.As depicted in step 410 in FIG. 11, the method 400 continues with step410 by calculating an X-directional offset value by the X-directionaldeviation (ΔX). The X-directional offset value is the reverse of theX-directional deviation (ΔX), or any value appropriate for adjusting theX-directional deviation (ΔX).

FIG. 17A is a cross-sectional view taken along a cutting line G-G′ ofFIG. 7, including an intensity I of an incident light and intensities I3and I4 of a first order diffraction. As depicted in FIG. 17A and step412 in FIG. 11, the method 400 continues with step 412 by measuring aY-directional deviation (ΔY) between the third alignment segments 320a-320 j and the first alignment segments 330 a-330 j. The overlaymeasurement principle used here is based on measuring asymmetry via thedetection of first or higher order diffraction. In some embodiments,step 412 may comprise several sub-steps 412 d, 412 e, and 412 f. FIG.18A shows the detail of sub-steps 412 d-412 f of step 412 in FIG. 11. Asdepicted in FIG. 17A, and sub-steps 412 d-412 f in FIG. 18A, anintensity I₃ of a positive first order diffraction on gratings of thethird alignment segments 320 a-320 j and the first alignment segments330 a-330 j is detected. An intensity I₄ of a negative first orderdiffraction on gratings of the third alignment segments 320 a-320 j andthe first alignment segments 330 a-330 j is also detected. An intensitydifference of I₃ and I₄ on gratings of the third alignment segments 320a-320 j and the first alignment segments 330 a-330 j is calculated,wherein the Y-directional deviation (ΔY) is proportional to theintensity difference.

If gratings of the third alignment segments 320 a-320 j and the firstalignment segments 330 a-330 j were on top of each other, forming onegrating, the intensity I₃ of the positive first order diffraction andthe intensity I₄ of the negative first order diffraction would be thesame, then there would be no Y-directional deviation (ΔY) between thethird alignment segments 320 a-320 j and the first alignment segments330 a-330 j. If there is any misalignment between these two gratings,this will induce an asymmetry A′ (i.e. the intensity difference) betweenthe intensities of I₃ and I₄, which is proportional to the Y-directionaldeviation (ΔY). This relationship could be expressed as one equationA′=I₃-I₄=K′*ΔY. In order to determine ΔY, the proportionality factor K′needs to be determined first. To solve this problem, gratings areshifted with respect to each other by a distance d′. If there was no ΔY,there would be gratings with a relative shift +d′ yielding an asymmetryA₃=K′*d′ and gratings with a relative shift −d′ yielding an asymmetryA₄=−K′*d′. In the presence of ΔY, the relative shifts of gratings amountto ΔY+d′ and ΔY−d′, yielding asymmetries A₃=K′*(ΔY+d′) andA₄=K′*(ΔY−d′). With these two equations, one may findΔY=d′*(A₃+A₄)/(A₃-A₄). So the Y-directional deviation (ΔY) is obtained.As depicted in step 414 in FIG. 11, the method 400 continues with step414 by calculating a Y-directional offset value by the Y-directionaldeviation (ΔY). The Y-directional offset value is the reverse of theY-directional deviation (ΔY), or any value appropriate for adjusting theY-directional deviation (ΔY).

As depicted in step 416 in FIG. 11, the method 400 continues with step416 by using the X-directional offset value and/or the Y-directionaloffset value to compensate for an overlay error. The overlay error maycomprise the X-directional deviation (ΔX), the Y-directional deviation(ΔY), or the combination of both. After the overlay error iscompensated, the multi-layer overlay alignment accuracy (AA) is adjustedand will be implemented in next run of the photolithography process. Insome embodiments, the method 400 may further comprise repeating aphotolithography process if the overlay error is larger than anacceptable deviation. If the overlay error is larger than an acceptabledeviation, the alignment between the third feature 320 and the firstfeature 330, and/or the alignment between the third feature 320 and thesecond feature 340, do/does not reach the required accuracy.Consequently, the third feature 320 has to be removed, and thephotolithography process has to be repeated until the overlay error isno larger than the acceptable deviation.

In various embodiments, the method 400 may be used with the overlay mark305 of FIGS. 9, 10A, and 10B. As mentioned earlier, the detaileddescription of FIGS. 9, 10A, and 10B for the overlay mark 305 may referto that of FIGS. 7, 8A, and 8B for the overlay mark 300. Thus, themethod 400 used with overlay mark 305 may also refer to the method 400used with overlay mark 300.

The overlay mark and method of the present disclosure are not limited tobe used by a planar device on the substrate and can also be applied to anon-planar device such as a fin-like field effect transistor (FinFET).By using the overlay mark and method of the present disclosure, theaccuracy of multi-layer overlay alignment is enhanced. Area costrequired by the overlay mark of the present disclosure is lower thanthat of the prior approach since the features of different layers forchecking alignment accuracy are positioned on the same area of thesubstrate. Also, time consumed by the method of the present disclosureis less than that of the prior approach because the method collects andanalyzes the data which are really concerned. For example, theY-directional deviation may be more concerned than the X-directionaldeviation for checking the alignment accuracy between the first layerand the third layer, while the X-directional deviation may be moreconcerned than the Y-directional deviation for checking the alignmentaccuracy between the second layer and the third layer. So, the methodtends to focus on the X-directional deviation between the second layerand the third layer, and the Y-directional deviation between the firstlayer and the third layer. As a result, the area cost and the timeconsumption can be reduced by using the overlay mark and method of thepresent disclosure.

One of the broader forms of the present disclosure involves an overlaymark. The overlay mark comprises a first feature extending only along afirst longitudinal direction; a second feature extending along a secondlongitudinal direction different from the first longitudinal direction;and a third feature extending along both the first longitudinaldirection and the second longitudinal direction; wherein the firstfeature is in a first layer, the second feature is in a second layerover the first layer, and the third feature is in a third layer over thesecond layer.

In some embodiments, the first longitudinal direction is substantiallyperpendicular to the second longitudinal direction.

In some embodiments, the first feature comprises a plurality ofalignment segments substantially parallel to each other.

In some embodiments, the second feature comprises a plurality ofalignment segments substantially parallel to each other.

In some embodiments, the third feature comprises a plurality ofalignment segments substantially parallel to each other extending alongthe first longitudinal direction and a plurality of alignment segmentssubstantially parallel to each other extending along the secondlongitudinal direction.

In some embodiments, the third feature comprises four alignment segmentsforming a rectangular pattern or a square pattern.

In some embodiments, the first feature comprises an isolation structure.

In some embodiments, the second feature comprises a gate electrode.

In some embodiments, the third feature comprises an opening in a mask.

Another of the broader forms of the present disclosure involves adevice. The device comprises a substrate; and an overlay mark over thesubstrate, wherein the overlay mark comprises a first feature extendingonly along a first longitudinal direction; a second feature extendingalong a second longitudinal direction different from the firstlongitudinal direction; and a third feature extending along the firstlongitudinal direction and the second longitudinal direction; whereinthe first feature is in a first layer, the second feature is in a secondlayer over the first layer, and the third feature is in a third layerover the second layer.

In some embodiments, the first longitudinal direction is substantiallyperpendicular to the second longitudinal direction.

In some embodiments, the first feature comprises a plurality ofalignment segments substantially parallel to each other.

In some embodiments, the second feature comprises a plurality ofalignment segments substantially parallel to each other.

In some embodiments, the third feature comprises a plurality ofalignment segments substantially parallel to each other extending alongthe first longitudinal direction and a plurality of alignment segmentssubstantially parallel to each other extending along the secondlongitudinal direction.

Still another of the broader forms of the present disclosure involves amethod of adjusting multi-layer overlay alignment. The method comprisesproviding a first feature in a first layer over a substrate, wherein thefirst feature comprises a plurality of first alignment segmentssubstantially parallel to each other extending only along an Xdirection; providing a second feature in a second layer over the firstlayer, wherein the second feature comprises a plurality of secondalignment segments substantially parallel to each other extending alonga Y direction different from the X direction; providing a third featurein a third layer over the second layer, wherein the third featurecomprises a plurality of third alignment segments substantially parallelto each other extending along the X direction and a plurality of fourthalignment segments substantially parallel to each other extending alongthe Y direction; measuring an X-directional deviation between the fourthalignment segments and the second alignment segments; calculating anX-directional offset value by the X-directional deviation; measuring aY-directional deviation between the third alignment segments and thefirst alignment segments; calculating a Y-directional offset value bythe Y-directional deviation; and using the X-directional offset value orthe Y-directional offset value to compensate for an overlay error.

In some embodiments, the step of measuring an X-directional deviationbetween the fourth alignment segments and the second alignment segmentscomprises defining a centerline X1 parallel to the fourth alignmentsegments; defining a centerline X2 parallel to the second alignmentsegments; and calculating the X-directional deviation between thecenterline X1 and the centerline X2.

In some embodiments, the step of measuring a Y-directional deviationbetween the third alignment segments and the first alignment segmentscomprises defining a centerline Y1 parallel to the third alignmentsegments; defining a centerline Y2 parallel to the first alignmentsegments; and calculating the Y-directional deviation between thecenterline Y1 and the centerline Y2.

In some embodiments, the step of measuring an X-directional deviationbetween the fourth alignment segments and the second alignment segmentscomprises detecting an intensity I₁ of a positive first orderdiffraction on gratings of the fourth alignment segments and the secondalignment segments; detecting an intensity I₂ of a negative first orderdiffraction on gratings of the fourth alignment segments and the secondalignment segments; and calculating an intensity difference of I₁ and I₂on gratings of the fourth alignment segments and the second alignmentsegments, wherein the X-directional deviation is proportional to theintensity difference.

In some embodiments, the step of measuring a Y-directional deviationbetween the third alignment segments and the first alignment segmentscomprises detecting an intensity I₃ of a positive first orderdiffraction on gratings of the third alignment segments and the firstalignment segments; detecting an intensity I₄ of a negative first orderdiffraction on gratings of the third alignment segments and the firstalignment segments; and calculating an intensity difference of I₃ and I₄on gratings of the third alignment segments and the first alignmentsegments, wherein the Y-directional deviation is proportional to theintensity difference.

In some embodiments, the method further comprises repeating aphotolithography process if the overlay error is larger than anacceptable deviation.

The foregoing has outlined features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. An overlay mark, comprising: a first featureextending only along a first longitudinal direction; a second featureextending along a second longitudinal direction different from the firstlongitudinal direction; and a third feature extending along the firstlongitudinal direction and the second longitudinal direction, the thirdfeature being continuous between the first longitudinal direction andthe second longitudinal direction; wherein the first feature is in afirst layer, the second feature is in a second layer over the firstlayer, and the third feature is in a third layer over the second layer,and the third feature is an opening in the third layer defining a mask.2. The overlay mark of claim 1, wherein the first longitudinal directionis substantially perpendicular to the second longitudinal direction. 3.The overlay mark of claim 1, wherein the first feature comprises aplurality of alignment segments substantially parallel to each other. 4.The overlay mark of claim 1, wherein the second feature comprises aplurality of alignment segments substantially parallel to each other. 5.The overlay mark of claim 1, wherein the third feature comprises aplurality of alignment segments substantially parallel to each otherextending along the first longitudinal direction and a plurality ofalignment segments substantially parallel to each other extending alongthe second longitudinal direction.
 6. The overlay mark of claim 1,wherein the third feature comprises four alignment segments forming arectangular pattern or a square pattern.
 7. The overlay mark of claim 1,wherein the first feature comprises an isolation structure.
 8. Theoverlay mark of claim 1, wherein the second feature comprises a gateelectrode.
 9. A device, comprising: a substrate; and an overlay markover the substrate, wherein the overlay mark comprises: a first featureextending only along a first longitudinal direction; a second featureextending along a second longitudinal direction different from the firstlongitudinal direction; and a third feature extending along both thefirst longitudinal direction and the second longitudinal direction, thethird feature being continuous between the first longitudinal directionand the second longitudinal direction; wherein the first feature is in afirst layer, the second feature is in a second layer over the firstlayer, and the third feature is in a third layer over the second layer,and the third feature is an opening in the third layer defining a mask.10. The device of claim 9, wherein the first longitudinal direction issubstantially perpendicular to the second longitudinal direction. 11.The device of claim 9, wherein the first feature comprises a pluralityof alignment segments substantially parallel to each other.
 12. Thedevice of claim 9, wherein the second feature comprises a plurality ofalignment segments substantially parallel to each other.
 13. The deviceof claim 9, wherein the third feature comprises a plurality of alignmentsegments substantially parallel to each other extending along the firstlongitudinal direction and a plurality of alignment segmentssubstantially parallel to each other extending along the secondlongitudinal direction.
 14. A method of adjusting multi-layer overlayalignment, comprising: providing a first feature in a first layer over asubstrate, wherein the first feature comprises a plurality of firstalignment segments substantially parallel to each other extending onlyalong an X direction; providing a second feature in a second layer overthe first layer, wherein the second feature comprises a plurality ofsecond alignment segments substantially parallel to each other extendingalong a Y direction different from the X direction; providing a thirdfeature in a third layer over the second layer, wherein the thirdfeature comprises a plurality of third alignment segments substantiallyparallel to each other extending along the X direction and a pluralityof fourth alignment segments substantially parallel to each otherextending along the Y direction, the third feature being continuousbetween the X direction and the Y direction; measuring an X-directionaldeviation between the fourth alignment segments and the second alignmentsegments; calculating an X-directional offset value by the X-directionaldeviation; measuring a Y-directional deviation between the thirdalignment segments and the first alignment segments; calculating aY-directional offset value by the Y-directional deviation; and using theX-directional offset value or the Y-directional offset value tocompensate for an overlay error, wherein the third feature is an openingin the third layer defining a mask.
 15. The method of claim 14, whereinthe step of measuring an X-directional deviation between the fourthalignment segments and the second alignment segments comprises: defininga centerline X1 parallel to the fourth alignment segments; defining acenterline X2 parallel to the second alignment segments; and calculatingthe X-directional deviation between the centerline X1 and the centerlineX2.
 16. The method of claim 14, wherein the step of measuring aY-directional deviation between the third alignment segments and thefirst alignment segments comprises: defining a centerline Y1 parallel tothe third alignment segments; defining a centerline Y2 parallel to thefirst alignment segments; and calculating the Y-directional deviationbetween the centerline Y1 and the centerline Y2.
 17. The method of claim14, wherein the step of measuring an X-directional deviation between thefourth alignment segments and the second alignment segments comprises:detecting an intensity I₁ of a positive first order diffraction ongratings of the fourth alignment segments and the second alignmentsegments; detecting an intensity I₂ of a negative first orderdiffraction on gratings of the fourth alignment segments and the secondalignment segments; and calculating an intensity difference of I₁ and I₂on gratings of the fourth alignment segments and the second alignmentsegments, wherein the X-directional deviation is proportional to theintensity difference.
 18. The method of claim 14, wherein the step ofmeasuring a Y-directional deviation between the third alignment segmentsand the first alignment segments comprises: detecting an intensity I₃ ofa positive first order diffraction on gratings of the third alignmentsegments and the first alignment segments; detecting an intensity I₄ ofa negative first order diffraction on gratings of the third alignmentsegments and the first alignment segments; and calculating an intensitydifference of I₃ and I₄ on gratings of the third alignment segments andthe first alignment segments, wherein the Y-directional deviation isproportional to the intensity difference.
 19. The method of claim 14,further comprising repeating a photolithography process if the overlayerror is larger than an acceptable deviation.